In a prior art circuit described in "VALVO Entwicklungsmitteilungen 78" three separate, parallel operating integrated circuits, each comprising an identification circuit are provided for the reception of signals in accordance with the PAL-standard, the SECAM-standard and the NTSC-standard.
In the circuit for the PAL-signal a flip-flop is used which is periodically triggered by pulses of the line frequency and produces a square-wave signal of half the line frequency and whose phase position is compared with the signal obtained by demodulation of the colour synchronizing signal which changes its phase position with the same period. A correcting circuit tries to adjust the two square-wave oscillations to equal phase positions. When this is accomplished (which is of course based on the assumption that a PAL signal is received) a d.c. voltage which serves as an identification signal is produced with which the relevant chrominance signal processing circuit, the colour killer in particular, is moved to the required switching condition.
In the circuit for the SECAM-signal a signal derived from frequency reference values of the chrominance signal and having half the line frequency is compared in a phase discriminator with a locally produced signal of the same frequency, and adjusted to the correct phase position. When a SECAM-signal is received, a predetermined d.c. voltage value occurs which ensures that the required switching actions are performed in the chrominance signal processing circuit.
In the circuit for the NTSC-signal the presence or the absence of colour synchronizing signals with the associated frequency of, for example, 3.58 MHz is used for a standard-depending control.
So the received colour television signal is applied in parallel to the three identification circuits and demodulation and passing the signal on is effected in that circuit which corresponds to the standard of the received television signal. This requires a considerable design effort and cost. Each identification circuit must be a complete unit capable of independent operation, as they must operate in parallel. As for the purpose of identification the colour synchronizing signal is sampled, which signal is available only for a short period of time and at the line frequency or, in SECAM, also at the field frequency, a storage capacitor must be provided for storing each voltage to be used for each standard change-over, so that if there are, for example, four standards four special capacitors are required, which because of their dimensions must be arranged outside an integrated circuit, so that the integrated circuit must be provided with four special connection terminals.